It is often desired to simulcast signals, i.e., concurrently transmit multiple signals, from a plurality of antenna elements comprising an antenna array (it shall be appreciated that as discussed herein, the antenna elements of an antenna array may in fact be any portion of an antenna structure producing a predefined radiation pattern when energized). Such simulcasting of signal is common, for example, in a phased array where each of the signals as provided to one of the antenna elements progresses in phase such that the energy radiated from all of the antenna elements combines and/or cancels to form a desired radiation pattern. Likewise, in a multibeam system, where individual predefined antenna beams are provided from an antenna array, simulcasting of signals, such as a control channel, over a plurality of the individual antenna beams so as to provide the signal in an area larger or differently shaped than that of an individual antenna beam, may be desired.
However, in the current state of the art, transmission of the aforementioned signals typically require a considerable amount of circuitry disposed between the transmitter and the antenna array. This circuitry may include significant lengths of transmission cable to carry the signal from the transmitter up the antenna mast to the antenna array. Additionally, active circuitry, such as filters, amplifiers, combiners, and the like may be disposed in the signal path to provide desired manipulation of the signals. This circuitry typically affects the transmitted signals in respects other than intended or desired.
For example, the lengths of cables associated with individual signals to be simulcast from an array may not be precise. Accordingly, a phase relationship, or phase progression, between the signals, initially introduced to provide a desired radiation pattern from the array, may be affected and thus nulls or other undesired effects in the combined radiation pattern may result.
Likewise, other circuitry, such as linear power amplifiers (LPA) disposed in the signal path may affect the desired phase relationship causing undesired results in the combined radiation pattern. Moreover, such circuitry may introduce cross coupling between the individual signals. For example, where a distributed amplifier is utilized, there is typically cross coupling between each of the input signals amplified. This cross coupling may affect the phase relationship in a non-linear or unpredictable manner. Therefore, it is difficult, if not impossible, to properly tune the signal circuits in order to maintain the desired phase relationships in advance or in a permanent fashion.
However, if the proper phase relationships are not maintained with respect to signals simulcast over multiple antenna elements, the combined radiation pattern may include the aforementioned nulls caused by destructive combining of signals. Present calibration techniques typically require the use of a probe, drone, or repeater communication unit to be placed in the radiation pattern of the antenna structure so as to provide information with respect to phase of the signals. One such system is disclosed in U.S. Pat. No. 5,546,090 issued to Roy. However, such techniques are undesirable as they require the deployment, maintenance, and expense of a transponder external to the antenna and transmission system being calibrated. The external transponder is an active component physically separate from, and often inconveniently located, causing additional expense in calibrating, servicing and testing such systems.
Accordingly, a need exists in the art for a fully self-contained, i.e., not external to the transmission and antenna circuitry, system and method for calibrating a plurality of signals to be simulcast so as to provide a desired phase relationship when simulcast.
A further need exists in the art for a system and method adapted to calibrate a plurality of signals to be simulcast which compensates for the existence of cross coupling or cross talk resulting from other signals.
A still further need exists in the art for any active components utilized in the calibration of signals to be disposed conveniently and securely with other active components of the transmission system.
A yet further need exists in the a t for the calibration system and method which operates automatically to dynamically calibrate a plurality of signals.